1. Field of the Invention
The present invention relates to a phase-shifting mask, and in particular to a structure and method of correcting proximity effects in a tri-tone attenuated phase-shifting mask.
2. Description of the Related Art
Photolithography is a well-known process used in the semiconductor industry to form lines, contacts, and other known structures in integrated circuits (ICs). In conventional photolithography, a mask (or a reticle) having a pattern of transparent and opaque regions representing such structures in one IC layer is illuminated. The emanating light from the mask is then focused on a photoresist layer provided on a wafer. During a subsequent development process, portions of the photoresist layer are removed, wherein the portions are defined by the pattern. In this manner, the pattern of the mask is transferred to or printed on the photoresist layer.
However, diffraction effects at the transition of the transparent regions to the opaque regions can render these edges indistinct, thereby adversely affecting the resolution of the photolithography process. Various techniques have been proposed to improve the resolution. One such technique, phase-shifting, uses phase destructive interference of the waves of incident light. Specifically, phase-shifting shifts the phase of a first region of incident light waves approximately 180 degrees relative to a second, adjacent region of incident light waves to create a more sharply defined interface between the first and second regions. Thus, the boundary between exposed and unexposed portions of a photoresist illuminated through a semiconductor mask (or reticle) can be more closely defined by using phase-shifting, thereby allowing greater structure density on the IC.
FIG. 1 illustrates a simplified, phase-shifting mask 100 fabricated with an attenuated, phase-shifting region 101 formed on a clear region 102, wherein a border 103 of attenuated, phase-shifting region 101 defines a single IC structure. Clear region 102 is transparent, i.e. a region having an optical intensity transmission coefficient T greater than 0.9. In contrast, attenuated phase-shifting region 101 is a partially transparent region, i.e. a region having a low optical intensity transmission coefficient 0.03 less than T less than 0.1. The phase shift of light passing through attenuated phase-shifting region 101 relative to light passing through clear region 102 is approximately 180 degrees.
As known by those skilled in the art, increasing the intensity transmission coefficient of attenuated phase-shifting region 101 could increase the performance of structures formed by the photolithographic process. In fact, optimal performance would be theoretically achieved by providing an attenuated, phase-shifting region with an optical intensity transmission coefficient approaching T=1 (in other words, the region would be transparent) yet having a phase shift of 180 degrees relative to clear region 102. In this manner, assuming partially coherent illumination, amplitude side lobes from each region would substantially cancel, thereby creating a substantially zero-intensity line at the transition between these two regions. Current material technology typically provides this phase shift with an attenuated, phase-shifting region having an optical intensity transmission coefficient of approximately T=0.4, although a higher transmission is theoretically possible and preferable.
Unfortunately, the use of this higher transmission phase-shifting material increases the risk of printing certain portions of attenuated phase-shifting region 101. Specifically, to ensure complete removal of residual photoresist, the actual dose used to remove the photoresist is typically at least twice the theoretical dose needed to remove the photoresist. This over-exposure can result in increasing the risk of printing certain larger portions of attenuated phase-shifting region 101.
To solve this problem, some masks, called tri-tone attenuated phase-shifting masks, include an opaque region within the larger portion(s) of the attenuated, phase-shifting region, wherein the opaque region blocks any unwanted light transmitted by the attenuated phase-shifting region. FIG. 2 illustrates a simplified, phase-shifting mask 200 fabricated with an attenuated phase-shifting region 201 formed on a clear region 202 and an opaque region 204 formed on attenuated phase-shifting region 201, wherein a border 203 of attenuated phase-shifting region 201 defines a single IC structure. In this embodiment, clear region 202 has an optical intensity transmission coefficient T greater than 0.9, attenuated phase-shifting region 201 has an optical intensity transmission coefficient 0.3 less than T less than 0.9, and an opaque region 204 typically has an intensity transmission coefficient of T less than 0.01. Note that the phase shift of light passing through attenuated phase-shifting region 201 relative to light passing through clear region 202 remains approximately 180 degrees.
Thus, forming an opaque region on an attenuated phase-shifting region advantageously allows for the use of a significantly higher optical intensity transmission coefficient for isolated structures. Unfortunately, a tri-tone phase-shifting mask exhibits strong optical proximity effects, thereby making it difficult to utilize this mask in a single common exposure for both isolated as well as crowded patterns.
Therefore, a need arises for a structure and a method for correcting optical proximity effects on a tri-tone, attenuated phase-shifting mask.
A structure and method are provided for correcting the optical proximity effects on a tri-tone phase-shifting mask. A tri-tone phase-shifting mask typically includes a plurality of attenuated phase-shifting regions formed on a transparent layer as well as opaque regions formed on the larger portions of the attenuated, phase-shifting regions to block any unwanted light transmitted by the attenuated phase-shifting regions. In this manner, the opaque regions prevent these larger portions of the attenuated phase-shifting regions from printing during the development process.
In accordance with the present invention, a rim, formed by an opaque region and an attenuated phase-shifting region, is kept at a predetermined width either for all structures of a certain type or for all structures across the mask. Typically, the rim is made as large as possible to maximize the effect of the attenuated phase-shifting region while still preventing the printing of larger portions of the attenuated phase-shifting region during the development process.
In accordance with one feature of the present invention, a photolithographic mask includes a plurality of structures. Some of the structures are formed with a transparent region, an opaque region, and an attenuated region. The opaque region and the attenuated region form an attenuated rim having a predetermined width. In the present invention, the width of this rim is substantially the same in the subset of structures.
In one embodiment, the transparent region provides approximately a 0 degree phase shift and has an optical intensity transmission coefficient greater than approximately 0.9, whereas the attenuated region provides approximately a 180 degree phase shift and has an optical intensity transmission coefficient between approximately 0.3 and approximately 1.0. In this embodiment, the opaque region has an optical intensity transmission coefficient of less than approximately 0.01.
In accordance with another feature of the present invention, a method of forming a plurality of structures in a tri-tone attenuated phase-shifting mask is provided. A subset of the structures are formed by a first region and a second region, wherein the first region has a phase shift relative to the second region of 180 degrees. The method includes positioning a third region within a boundary for the second region, thereby forming a rim of the second region. The third region prevents the second region from printing during the development process. In accordance with the invention, a predetermined rim width for the subset of structures is provided to correct for optical proximity.
In one embodiment, the first region includes a transparent region, the second region includes an attenuated region, and the third region includes an opaque region. In this embodiment, the method further includes dividing the border for the second region into a plurality of first segments, each first segment including two dissection points. A subset of the dissection points is projected onto a border for the third region, thereby forming a plurality of second segments. At this point, optical proximity correction is provided for the subset of structures, wherein if an optical proximity correction moves a first segment, then a corresponding second segment moves. A modified border for the third region is determined based on a modified second region formed after optical proximity correction.
In one embodiment, determining the modified border for the third region includes downsizing the modified second region and then upsizing the downsized second region. For example, the modified second region can be downsized by twice the predetermined rim width and then upsized by the predetermined rim width. In another example, the modified second region is downsized by the predetermined rim width and then any resulting mousebites in the downsized second region are eliminated. In yet another example, the amount of downsizing and subsequent upsizing can be adjusted as a function of the optical proximity correction applied (i.e. size of the hammerhead, bias, serif, etc.). In yet another example, the amount of downsizing and subsequent upsizing takes into account any side-lobe printing that could occur inside the structure.
In another embodiment of the invention, optical proximity correction is provided for the subset of structures, however the border for the third region is not changed during this correction. Once again, the modified border for the third region is determined based on a modified second region formed after optical proximity correction. In one embodiment, the modified border for the third region is determined by downsizing the modified second region and then upsizing the downsized second region. For example, the modified second region can be downsized by twice the predetermined rim width and then upsized by the predetermined rim width. In another example, the amount of downsizing and subsequent upsizing can be adjusted as a function of the optical proximity correction applied (i.e. size of the hammerhead, bias, serif, etc.). In yet another example, the amount of downsizing and subsequent upsizing takes into account any side-lobe printing that could occur inside the structure. In yet another example, the modified second region is downsized by the predetermined rim width and then any resulting mousebites in the downsized second region are eliminated.
In accordance with another feature of the present invention, a method of fabricating a tri-tone attenuated phase-shifting mask includes the steps of forming a transparent layer and an attenuated layer, wherein a phase shift of the attenuated layer relative to the transparent layer is approximately 180 degrees. The attenuated layer is patterned, wherein a transition from a transparent portion to an attenuated portion defines an edge of a structure on the mask. An opaque layer is formed and patterned, wherein for each structure on the mask including an opaque portion, the opaque portion is located a predetermined distance from the edge.
The patterning of the attenuated and opaque layers is preceded by simulating optical proximity correction for the structures. In one embodiment, a transition from the opaque portion to the attenuated portion defines an edge of a rim, wherein simulating optical proximity correction includes moving segments of the edge of the rim if corresponding segments of the edge of the structure move. At this point, the attenuated portion can be downsized and then upsized. In one example, the attenuated portion can be downsized by twice the predetermined distance and then upsizing by the predetermined distance. In another example, the attenuated portion is downsized by the predetermined distance. In yet another example, the amount of downsizing and subsequent upsizing can be adjusted as a function of the optical proximity correction applied (i.e. size of the hammerhead, bias, serif, etc.). In yet another example, the amount of downsizing and subsequent upsizing takes into account any side lobe printing that could occur inside the structure.
In another embodiment, simulating optical proximity correction includes moving segments of the edge of the structure while fixing the edge of the rim. The attenuated portion can be downsized and then upsized. As indicated above, the downsizing and upsizing can be based on predetermined distances, a function of optical proximity correction, or adjusted to minimize any internal side lobe printing.
In accordance with another feature of the invention, a semiconductor mask comprises a plurality of structures. A subset of the structures includes a first region having an optical intensity transmission coefficient greater than 0.9, a second region having an optical intensity transmission coefficient of less than 0.01, and a third region having an optical intensity transmission coefficient between approximately 0.3 and approximately 1.0. In the present invention, the second region and the third region form a rim having a predetermined width, wherein the width is substantially the same in the subset of structures.
In accordance with another feature of the present invention, computer software for simulating a tri-tone attenuated phase-shifting mask is provided. The mask includes a plurality of structures, a subset of the structures including a transparent region, an opaque region, and an attenuated region. The opaque region and the attenuated region form a rim. The computer software includes means for analyzing optical proximity correction for the subset of the structures and means for providing a substantially similar rim width in the subset of the structures. In one embodiment, the means for providing includes means for dividing a first edge of the attenuated region into a plurality of first segments, means for dividing a second edge of the opaque region into a plurality of second segments, wherein each second segment corresponds to a certain first segment, and means for determining whether a second segment moves with its corresponding first segment during optical proximity correction. The means for providing can also include means for downsizing the attenuated region and then upsizing the attenuated region to generate the substantially similar rim width. Alternatively, the means for providing can include means for downsizing the attenuated region to generate the substantially similar rim width.